Polymer electrolyte fuel cells using a proton-conductive solid polymer membrane are expected to serve as a power source of moving vehicles such as cars and are beginning to be put into practice since they can operate even at a low temperature compared to other fuel cells such as solid oxide fuel cells and fused carbonate fuel cells.
A gas diffusing electrode used in polymer electrolyte fuel cells includes an electrode catalyst layer containing carbon-supported catalyst particles coated with the same or a different ion-exchange resin (polymer electrolyte) from their polymer electrolyte membrane and a gas diffusion layer configured to supply a reactant gas to the catalyst layer and also to collect electric charge generated in the catalyst layer. A membrane electrode assembly (MEA) is formed by joining such a gas diffusion layer to a polymer electrolyte membrane with its catalyst layer facing the polymer electrolyte membrane. A polymer electrolyte fuel cell is formed by stacking a plurality of such membrane electrode assemblies with intervening separators including a gas channel.
One of the gas diffusion electrodes used in polymer electrolyte fuel cells known in the art includes a micro porous layer as an intermediate layer for decreasing the electric resistance between the gas diffusion layer and the catalyst layer and improving gas flow. The micro porous layer is mainly made of an electrically conductive material such as a carbon material and is disposed at the catalyst layer side of the gas diffusion layer.
Japanese Patent Unexamined Publication No. 2007-273457 discloses a method for producing such a polymer electrolyte fuel cell that involves applying a water-repellent layer composition to a gas diffusion layer so as to form a water-repellent layer, forming a catalyst electrode layer on the water-repellent layer and/or a solid polymer electrolyte membrane, and thereafter bonding the gas diffusion layer to the electrolyte membrane by means of thermal compression bonding with intervening the water repellent layer and the catalyst electrode layer therebetween. The gas diffusion layer is made of carbon fiber, and the water-repellent layer composition contains a water-repellent material such as fluororesin, an electrically conductive material such as carbon black, and a shape retaining material such as carbon fiber. The water-repellent layer formed from the composition corresponds to a micro porous layer.
However, in the production method disclosed in Japanese Patent Unexamined Publication No. 2007-273457, it is required to apply an excessive pressure during the thermal compression bonding of the solid polymer electrolyte membrane to the gas diffusion layer since the gas diffusion layer is compressed and deformed. Accordingly, the carbon fiber may dig into the electrolyte membrane to cause damage. Furthermore, applying a high pressure and a heat requires large equipment, which increases the production cost.